The present invention relates to a means and a method for learning to determine whether a condition which is definable in terms of detectable numerical quantities is normal or abnormal, and more particularly, to such means for determining whether the condition for VLF (Very Low Frequency) wave propagation is normal or abnormal.
The use of VLF, or radio waves in the 3-30 kHz range, is finding increasing application in navigation systems. Generally, in VLF a vehicle determines its position at a given time by employing a phase tracker and a reference signal provided by an oscillator to determine the change in phase angle of a VLF signal generated by a radio station, as the vehicle moves from a known location to its location at the given time. A change in phase angle indicates the distance the vehicle has traveled toward or away from the station. By tracking the change in phase angle of the signals from each of two or three or more VLF stations a navigator aboard the vehicle may determine his position.
A propagated VLF wave may be conceptualized as a wave traveling through a spherical waveguide wherein one surface of the waveguide is the surface of the earth and the other surface is the upper limit of the lower ionosphere, or "D" region.
A major source of error in VLF navigation is caused by unpredictable solar disturbances, which may randomly send large numbers of X-rays or protons into the earth's atmosphere, effectively lowering the height of the D region. If the D region is lowered, the propagated VLF wave is traveling through a narrower waveguide so that its velocity must increase, whereby its phase angle is advanced. If a navigator was unaware of the advance in phase angle, he could unknowingly miscalculate his position.
A further error in VLF navigation may result from the unpredictable phenomenon of cycle slippage. Under some conditions for propagation of a VLF wave, one mode of propagation is dominant during the day, whereas a different mode is dominant during the night. During a period, such as during an evening, when the propagated wave is in transition from one dominant mode to the other, cycle slippage may occur wherein the VLF wave is anomalously advanced by an entire cycle. A phase tracker may accurately track the change in phase angle of the VLF wave without being aware of the anomalous cycle advance. Consequently, a position calculated on the basis of the phase angle change could be in error by a distance equal to a wavelength or to a half wavelength of the VLF wave. At 10 kHz this could be as much as sixteen miles when navigating against a reference derived from a precision clock. This distance could be very significant, such as when flying near mountains or in narrow flight lanes.
When solar and atmospheric conditions vary in an unpredictable manner, for example, when solar disturbance or anomalous cycle slippage occurs, the VLF propagation condition may be considered to be abnormal, since an unpredictable navigationally significant variation of the phase angle of the VLF wave may occur. The phase angle in a given region may be expected to vary over a period of time, of course, as the solar radiation in the region varies. If the variation of phase angle is cyclical, for example, the variation during a 24 hour diurnal cycle, so that it may be predictable, the VLF propagation condition may be considered to be normal, since the variation in phase angle may be measured and accounted for. Measurements of predictable variations of phase angle, however, may be very tedious, particularly if they require extensive manual observations and calculations at a number of sites.